[NET]: Add netif_tx_lock

Various drivers use xmit_lock internally to synchronise with their
transmission routines.  They do so without setting xmit_lock_owner.
This is fine as long as netpoll is not in use.

With netpoll it is possible for deadlocks to occur if xmit_lock_owner
isn't set.  This is because if a printk occurs while xmit_lock is held
and xmit_lock_owner is not set can cause netpoll to attempt to take
xmit_lock recursively.

While it is possible to resolve this by getting netpoll to use
trylock, it is suboptimal because netpoll's sole objective is to
maximise the chance of getting the printk out on the wire.  So
delaying or dropping the message is to be avoided as much as possible.

So the only alternative is to always set xmit_lock_owner.  The
following patch does this by introducing the netif_tx_lock family of
functions that take care of setting/unsetting xmit_lock_owner.

I renamed xmit_lock to _xmit_lock to indicate that it should not be
used directly.  I didn't provide irq versions of the netif_tx_lock
functions since xmit_lock is meant to be a BH-disabling lock.

This is pretty much a straight text substitution except for a small
bug fix in winbond.  It currently uses
netif_stop_queue/spin_unlock_wait to stop transmission.  This is
unsafe as an IRQ can potentially wake up the queue.  So it is safer to
use netif_tx_disable.

The hamradio bits used spin_lock_irq but it is unnecessary as
xmit_lock must never be taken in an IRQ handler.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 35 insertions, 3 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index b5760c67af9c..067b9ccafd87 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -407,7 +407,7 @@ struct net_device
  * One part is mostly used on xmit path (device)
  */
         /* hard_start_xmit synchronizer */
-        spinlock_t              xmit_lock ____cacheline_aligned_in_smp;
+        spinlock_t              _xmit_lock ____cacheline_aligned_in_smp;
         /* cpu id of processor entered to hard_start_xmit or -1,
            if nobody entered there.
          */
@@ -893,11 +893,43 @@ static inline void __netif_rx_complete(struct net_device *dev)
         clear_bit(__LINK_STATE_RX_SCHED, &dev->state);
 }
 
+static inline void netif_tx_lock(struct net_device *dev)
+{
+        spin_lock(&dev->_xmit_lock);
+        dev->xmit_lock_owner = smp_processor_id();
+}
+
+static inline void netif_tx_lock_bh(struct net_device *dev)
+{
+        spin_lock_bh(&dev->_xmit_lock);
+        dev->xmit_lock_owner = smp_processor_id();
+}
+
+static inline int netif_tx_trylock(struct net_device *dev)
+{
+        int err = spin_trylock(&dev->_xmit_lock);
+        if (!err)
+                dev->xmit_lock_owner = smp_processor_id();
+        return err;
+}
+
+static inline void netif_tx_unlock(struct net_device *dev)
+{
+        dev->xmit_lock_owner = -1;
+        spin_unlock(&dev->_xmit_lock);
+}
+
+static inline void netif_tx_unlock_bh(struct net_device *dev)
+{
+        dev->xmit_lock_owner = -1;
+        spin_unlock_bh(&dev->_xmit_lock);
+}
+
 static inline void netif_tx_disable(struct net_device *dev)
 {
-        spin_lock_bh(&dev->xmit_lock);
+        netif_tx_lock_bh(dev);
         netif_stop_queue(dev);
-        spin_unlock_bh(&dev->xmit_lock);
+        netif_tx_unlock_bh(dev);
 }
 
 /* These functions live elsewhere (drivers/net/net_init.c, but related) */